1. Field of the Invention
This invention pertains generally to bulk material belt conveyor systems. More particularly, this invention pertains to an apparatus and method of estimating the amount of carryback material that is transferred by the belt of such a conveyor system.
2. General Background Technology
Bulk material belt conveyor systems are utilized to transfer bulk materials from a location near the tail of the conveyor system to a location at the head of the conveyor system. Invariably, some bulk material transferred by the belt of a conveyor system sticks or adheres to belt rather than being dump from the belt at the head of the conveyor system and travels back toward the tail of the conveyor system. Such bulk material is referred to in the art as carryback material. Some conveyor systems include belt scrapers and/or water jets for the purpose of dislodging such carryback material. Such devices are typically positioned at or near where the belt begins its return path from the head portion to the tail portion of the conveyor system. However, even with the use of belt scrapers and/or water jets, typically not all of the carryback material can be removed from the belt. The amount of carryback material that remain on a belt varies with such things as the moisture content of the bulk material, the physical properties of the bulk material, the temperature, and the condition of the belt and scrapers.
In some cases, it is desirable or even a requirement to estimate the amount of carryback material being transferred by the belt of a conveyor system. Such an estimate is often performed by temporarily engaging the return side of the belt with a carryback scraper for a duration of time and collecting the amount of carryback material removed by the scraper for measurement. In most cases, the amount of carryback material is dried using heat or a partial vacuum and then weighed. The width of carryback scrapper, the speed of the belt, the duration of time, and the mass of the dried carryback martial is then used to approximate such things as the mass of carryback material per area of belt or the mass of carryback material transferred by the belt per unit time. Typically the approximation is given in units of grams per square meter of belt surface in contact with the bulk material.
Different devices exist for estimating the amount of carryback material being transferred by the belt of a conveyor system. One such device developed by Hosch Fordertechnik GmbH (HOSCH) uses a carryback sampling device that comprises a carryback scraper mounted on a shaft that extends across the width of the conveyor beneath the belt. The scraper is attached to the shaft and, with the belt operating, the shaft is rotated to point where the carryback scraper makes contact with the downwardly facing surface of the belt. A bucket is also supported by the shaft and is positioned beneath the carryback scraper. As this occurs, carryback material is removed from the belt via the carryback scraper and falls into the bucket. After this occurs for a period of time, the belt operation is stopped and the bucket, along with its contents of carryback material are removed from under the belt. The carryback material is then typically dried and weighed (usually offsite), and a determination of the amount of carryback transferred by the belt is determined based on the weight of the measure carryback material. A drawback of this device is that the belt must be stopped in order to install the scraper and the bucket and to recover a carryback sample and reset the device to take additional samples. This reduces the productivity of the conveyor system. Another drawback of the HOSCH device is that the pressure exerted by the carryback scraper against the belt is dependent on the elevation of the shaft and the condition of or wear on the carryback scraper, which inherently change from one test to the next. Thus, the HOSCH device does not provide a consistent means of measuring the carryback material transferred by a conveyor system.
Another device for sampling carryback material known in the industry is the ICT Automated Carryback Monitor (ICT) developed by the University of Newcastle and used by several companies such as ESS Engineering Services and Supplies in Corrumbin, Australia. The ICT device comprises a shaft extending across the width of a conveyor beneath the belt. The shaft is supported from the support frame of the conveyor system via actuators that are configured to pneumatically or hydraulically raise the shaft from a lowered position to a raised position relative to the belt. A carryback material receptacle is mounted on shaft as are a plurality of carryback scrapers that extend upward. The ICT device also comprises a motor and chain system that is configured to reciprocate the carryback receptacle and the carryback scrapers back and forth along the shaft. In operation, the ICT device raises the shaft into the raised position, where the carryback scrapers engage the downward facing surface of the belt, and reciprocates the carryback receptacle and the carryback scrapers back and forth along the shaft for a specific time while the belt is operating. During such time, carryback material is scraped from the belt in a serpentine pattern via the carryback scrapers and deposited into the carryback material receptacle. The reciprocation of the carryback scrapers and the carryback material receptacle along the shaft is performed to yield an accurate reflection of the average carryback material across the width of the belt. After the specific amount of time has passed, the shaft is automatically lowered, thereby disengaging the carryback scrapers from the belt, and the motor and chain drive the carryback material receptacle out from under the belt to one side of shaft for removal. As was the case with using the HOSCH device, the carryback material collected by the ICT device is typically dried and weighed. The ICT device is configured to obtain relatively accurate measurements of carryback material. However, the ICT device requires external power (electricity and pressurized air or oil). Moreover, although the belt does not need to be stopped in order to use the ICT carryback sampling device, the belt does have to be stopped in order to assemble The ICT carryback sampling device to a conveyor system. Additionally, the accuracy of measurements obtained by the ICT carryback sampling device is dependent upon the precise alignment of the carryback scrapers relative to their mounting in the carryback material receptacle, which is difficult to achieve.